1. Field
Aspects of the present invention generally relate to an image forming system, a control method, and a program.
2. Description of the Related Art
When a printer does printing, the printer transfers toner to a sheet, heat-presses the toner using a fixing unit, and fixes the toner on the sheet.
In such a case, if a color solidly fills an image, moisture in the sheet rapidly evaporates while the sheet passes through the fixing unit. The toner positioned on an upstream side of the fixing unit is then partially blown off, so that an image failure referred to as tailing (i.e., toner scattering) occurs. To prevent such tailing, Japanese Patent Application Laid-Open No. 2004-314308 discusses a technique which performs thinning on raster data. Further, Japanese Patent Application Laid-Open No. 2000-066536 discusses a technique for preventing tailing (toner scattering) by setting a transfer voltage applied when transferring the toner to the sheet to a high voltage.
On the other hand, if a sheet type or a sheet size is special, the printer may perform printing by decreasing a conveyance speed of the sheet. For example, if the sheet type is cardboard, it is necessary to perform fixing for a long time, so that the conveyance speed is decreased. Further, if the sheet size is of a narrow width, temperature of edge portions of the fixing unit rise, so that the conveyance speed of the sheet is decreased.
The toner is transferred from a photosensitive member to the sheet as follows. The voltage of an opposite voltage as the toner is applied to a transfer member such as a transfer roller press-contacting or opposing the photosensitive member. An electric field is thus formed so that the toner is directed towards the transfer member. In such a case, a charge of opposite polarity to the toner is applied to the sheet, and the toner is held on the sheet in an electrostatic manner. In light of a holding power caused by static electricity, it is advantageous to apply a large amount of charge to the sheet so that the above-described tailing-blur does not easily occur. However, if the transfer voltage becomes excessively high, the charge overflows on the sheet and thus leaps over the sheet to reach the toner. The toner is thus charged to have the same polarity as the transfer voltage, and toner scattering and a transfer failure such as re-transfer occurs.
If the transfer voltage for transferring the toner to the sheet is set high to prevent tailing-blur, the transfer failure does not easily occur especially in the case where the conveyance speed of the sheet is low. More specifically, the amount of charge implanted in the sheet is determined by multiplying the transfer voltage by a passing time of the sheet. As a result, when the conveyance speed is low (i.e., the passing time is long), it becomes easier for the charge to overflow on the sheet even if the transfer voltage is the same.